Scientists almost never spot meteors, like the one that caused damage in Russia Friday, before they strike Earth.
"We virtually never get them ahead of time," said Margaret Campbell-Brown, associate professor in the department of physics and astronomy at Western University in London, Ont.
'It's only really good luck if we happen to see one before it hits the Earth.'—Margaret Campbell-Brown, Western University professor
"It's only really good luck if we happen to see one before it hits the Earth."
Scientists call such space debris near-Earth objects (NEOs) and have observed nearly 10,000 of them since NASA started using telescope tracking in 1995, according to NASA's NEO program website.
How did scientists spot asteroid 2012 DA14, the meteor that was recorded in real-time as it crossed within the orbit of the moon the same day the one above Russia came crashing down?
The answer was luck.
Scientists only knew about it because DA14 had already passed by our planet before, said Campbell-Brown.

A 'close shave'

Campbell-Brown explained that these NEOs "sort of hang out in the neighbourhood of the Earth," drawing nearer and farther from our planet as they loop around, following the path of their orbit.
NASA generally tracks NEOs that range in size from one to approximately two kilometres in diameter and continually orbit the Earth, some closer than others.
"Close," according to Campbell-Brown is anything that comes within four times the distance to the moon, or around 1.5 million kilometres. The professor described asteroids that fly within twice the distance to the moon as "a close shave."
It's a close shave because any NEO that passes within four times the distance to the moon "feels" the Earth's gravitational pull and therefore the closer it passes Earth the closer it comes to hitting our planet during its next pass.
NEOs less than 50 metres in diameter, such as the one that exploded over Russia, estimated to be about the size of a bus, are too hard to continually detect with modern telescopes, said Campbell-Brown. There are also too many of them around the Earth's orbit and they pass near the Earth too quickly.

Sonic booms for nuclear detonations

NEOs hit every couple of decades, said Campbell-Brown. But such events are extremely rare. Prior to Friday's incident in Russia, the largest recorded explosion of a space object plunging to Earth occurred in 1908, when a meteorite crashed in central Siberia and levelled some 80 million trees.
CBC science reporter Bob McDonald explained that when space rocks enter the Earth's atmosphere "and hits the air, it comes to a screeching halt, and the pressure of the air and the heat on the front side of it, compared to the back side, causes the whole thing to collapse in on itself, and it does that so quickly that there's just this massive air burst explosion."
These sonic booms are then picked up by Comprehensive Nuclear Test Band Treaty (CTBT) stations. Stations are set up by countries around the globe to detect nuclear detonations.
One of the ways the stations do that is by using infrasound. The state-of-the-art technology picks up ultra-low frequency sound waves created by the sonic boom (inaudible to the human ear).
The sonic boom over Russia was heard by at least two CTBT stations.
If a meteor is big enough, it will not only create a sonic boom but maintain what Campbell-Brown describes as "sonic speeds" and create a massive explosion, as opposed to slowing down to upon entering the atmosphere and make "a hole in the ground."
"If a large enough object hit the ground still going very quickly," she said. "It wouldn't be slowed down as much by the atmosphere and it might create this explosive impact crator and cause a lot of damage and throw a lot of dust in the atmosphere."
NASA's website states that a near-Earth object larger than one kilometre could cause a global disaster.
"The impact debris would spread throughout the Earth's atmosphere so that plant life would suffer from acid rain, partial blocking of sunlight, and from the firestorms resulting from heated impact debris raining back down upon the Earth's surface."

Time is of the essence

So what if NASA discovers a massive meteor on a collision course with Earth?
Campbell-Brown doesn't have a definitive answer.
"There certainly has been a lot of discussion about what governments, who should be in charge, if someone does discover something that will hit the Earth," the professor said. "Are they supposed to call the U.S. government? The UN?"
Timothy B. Spahr, director of the Minor Planet Data Centre, which is funded by NASA and monitors the impact probability of larger objects, shed more light on the subject.
"We have a plan in place if we have warning: I alert NASA headquarters and they pass the information up the line, which includes contacting the U.S. State Department, the White House, and folks at the UN, I'm sure," said Spahr.
"Obviously we'd like to have years of warning instead of seconds," added Spahr.
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Scientists struggle to predict meteorite strikes